\documentclass{article}
\usepackage{booktabs}
\begin{document}
  \begin{center}
    \begin{tabular}{llll}
      \toprule
      Type & \multicolumn{2}{l}{Register} & Purpose \\
      \midrule
      General Purpose & ax & ah & Arithmetic \\
      & & al & \\
      \addlinespace % \cmidrule{2-4}
      & bx & bh & Base \\
      &  & bl & \\
      \addlinespace
      & cx & ch & Counter \\
      & & cl & \\
      \addlinespace
      & dx & dh & Data \\
      & & dl & \\
      \cmidrule{2-4}
      & \multicolumn{2}{l}{sp} & Stack Pointer \\
      & \multicolumn{2}{l}{bp} & Base Pointer \\
      & \multicolumn{2}{l}{si} & Source Index \\
      & \multicolumn{2}{l}{di} & Destination Index \\
      \midrule
      Segment & \multicolumn{2}{l}{ss} & Stack Segment \\
      & \multicolumn{2}{l}{cs} & Code Segment \\
      & \multicolumn{2}{l}{ds} & Data Segment \\
      & \multicolumn{2}{l}{es} & Extra Segment \\
      & \multicolumn{2}{l}{fs} & Extra Segment \\
      & \multicolumn{2}{l}{gs} & Extra Segment \\
      \bottomrule
    \end{tabular}
  \end{center}
\end{document}
